Biosensors typically are devices that allow qualitatively or quantitatively detection of target molecules, also called “analytes”, such as e.g. proteins, cells, viruses, bacteria, protozoa, cell components, cell membranes, spores, DNA, RNA, etc. in a liquid, such as for example blood, serum, plasma, urine, saliva, tissue extract, interstitial fluid, cell-culture extract, food or feed extract, liquids such as drinking water, etc. One of the measuring principles is the counting of labelled molecules attached at predetermined sites on the biosensor. For example, the molecules may be labelled with magnetic particles or beads and these magnetic particles or beads can be detected with a magnetic sensor. Alternatively, the amount of analyte may be detected by optical methods such as fluorescence. In this case the analyte itself may carry a fluorescent label, or alternatively an additional incubation with a fluorescent labelled second recognition element may be performed. Some molecules to be analysed are self-fluorescent.
There are a number of label-based assays used in the analytical detection of a target. These include the sandwich, competitive and inhibition assays. In some formats for these assays the assay reaction must be separated in time from the detection of the label. In some assays it is desirable to first complete the assay reaction on the sensor surface and thereafter attach the label for detection. This sequence of reactions is often used for immunoassays in which a sandwich of the target between a primary and secondary antibody is first formed on the surface, e.g. using a biotinylated secondary antibody, and thereafter the label, e.g. containing streptavidin, is bound to the sensor through the sandwich. In other assays it is more effective to first complete the assay reaction on the label and then bring the label to the sensor for detection. Examples include the assays based on a strong-binding couple for the attachment of the label to the sensor surface. In this assay, a sandwich of the target between a primary and secondary antibody is first formed on the label and thereafter the label is bound on the sensor surface for detection. In such assays, it is desirable to prevent the unreacted reagents (e.g. reagents that have not reacted to form a sandwich) from binding to the label or sensor surface and then blocking the label from being detected. Methods for doing this include removing the reagents by washing the reaction vessel or binding them onto a reactive, non-sensor surface. Washing is inconvenient as it involves extra fluid handling steps and can also result in the removal of the labels to be detected. It is also difficult to avoid that the label also binds to the reactive surface intended for unreacted reagents.
Magnetic biosensors detect biological targets labeled with magnetic particles. US 2006/0011552 A1 discloses an apparatus and method for separating a predetermined amount of magnetic particles in a fluid. In the system, magnetic particles from a fluid are captured at a fluid conveyance portion. The latter is performed using a magnetic field at the fluid conveyance portion positioned at a cross-section of a fluid supply channel and a discharge channel. Thereafter, the magnetic particles are discharged by a way of a medium introduced into the second flow channel. In this way, a predetermined amount of magnetic particles is provided in a fluid.